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From the 1940s through the 1970s there was no major warming trend in the average surface temperature of Earth. At the same time, the tropical Pacific Ocean, which is responsible for the weather patterns known as El Niño and La Niña that can swing global average temperatures by as much as 0.3 degree Celsius, was anomalously cold. For the past decade or so the tropical Pacific has again gone cold—more Niña than Niño—and a new study suggests that the phenomenon may explain the recent "pause" in global warming of average temperatures.

Since 1998's record heat, average surface temperatures have plateaued for a decade or so—failing to reach new peaks—although the decade also qualifies as the hottest on record. Rising concentrations of greenhouse gases in the atmosphere have not accelerated warming to new heights as rapidly as happened at the end of the 20th century.

To explain this apparent hiatus, climate scientists Shang-Ping Xie and Yu Kosaka of the Scripps Institution of Oceanography at the University of California, San Diego, added the sea. Plugging in observed sea-surface temperatures as well as the more traditional numbers for the extra heat trapped by greenhouse gases into the U.S. Department of Commerce’s Geophysical Fluid Dynamics Laboratory computer model of the oceans and atmosphere might reveal if the cooler tropical Pacific was responsible for the climate change pause. By adding in the sea-surface temperatures of an oceanic area covering roughly 8 percent of the globe, the researchers were able to mimic the recent hiatus in global warming as well as weather phenomena like the prolonged drought in the southern U.S. The results are detailed in Nature on August 29. (Scientific American is part of Nature Publishing Group.) "The tropical Pacific is the engine that drives the global atmosphere and climate," Xie says. "There were epochs of accelerated and stalled warming in the past," including that pause in a global warming trend between the 1940s and 1970s, which has often been attributed to sunlight-blocking air pollution from Europe, the Soviet Union and the U.S.

Whereas the largest ocean on a globe that is 70 percent water covered is an obvious driver of climate patterns, it is less clear what drives the cycles of cooling and heating of tropical Pacific Ocean waters. But it is clear that the cool Pacific pattern cannot persist forever to cancel out the extra heat trapped by rising CO2 concentrations, Xie notes.

Other factors—volcanoes, an unusually weak solar cycle, air pollution from China—probably play an important role in restraining global warming as well. Some of the observed climate effects may also stem from other ocean dynamics such as variations in the mixing of surface and deep ocean waters. Already, it appears that ocean waters down to 2,000 meters in depth have trapped a disproportionate share of heat, warming by roughly 0.1 degree C (the equivalent of roughly 36 degrees C of atmospheric warming) since 1955. And the meltdown of significant ice from Greenland or Antarctica might even cool oceans enough to offset the extra heat trapped by rising levels of greenhouse gases for a time. "We need updates to the forcings and a proper exploration of all the different mechanisms together," says climate modeler Gavin Schmidt of the NASA Goddard Institute for Space Studies. "This has taken time but will happen soon-ish." The U.N. Intergovernmental Panel on Climate Change will deliver its update on global warming at the end of September.

Despite any pause in the trend toward hotter temperatures, the first decade of the 21st century was the hottest based on records kept since the 1880s—and it included record heat waves in Russia and the U.S. as well as a precipitous meltdown of Arctic sea ice and surging sea level rise. Atmospheric concentrations of CO2 touched 400 parts per million on Mauna Loa in May, a first in the time line of human existence. A cooler Pacific stuck in a La Niña rut may have restrained global warming for the past decade or so, Xie notes, but it is unlikely to last. "This effect of natural variability will be averaged out over a period of 100 years," he says, "and cannot argue away the threat of persistent anthropogenic warming that is occurring now."